Ferroelectric materials are characterized primarily by a spontaneous polarization, the orientation of which can be reversed by an electric field. In addition, these materials also display unique dielectric, pyroelectric, piezoelectric and electro-optic properties that are utilized for a variety of applications such as capacitors, dielectric resonators, heat sensors, transducers, actuators, nonvolatile memories, optical waveguides and displays. For device applications, however, it is useful to fabricate ferroelectric materials in the form of thin films so as to exploit these properties and the design flexibility of thin film geometries.
Device applications also require that the bulk properties of the ferroelectrics are achieved in the thin films and therefore it is necessary to employ a deposition technique that can provide optimum thin-film characteristics such as stoichiometry, crystallinity, density, microstructure and crystallographic orientation. Although a variety of deposition techniques have been used, the growth of the films with controlled properties at relatively low temperatures is still a challenge and several techniques are being explored to achieve this objective. In general, thin film deposition techniques can be classified into two major categories, i.e., (1) physical vapor deposition (PVD) and (2) chemical processes (see "The Materials Science of Thin Films", Milton Ohring, Academic Press, 1992; S. L. Swartz, IEEE Transactions on Electrical Insulation, 25(5), 1990, 935; S. B. Krupanidhi, J. Vac. Sci. Technol. A, 10(4), 1992, 1569). The chemical processes can further be divided into two subgroups i.e., chemical vapor deposition and wet chemical processes including sol-gel and metalorganic decomposition (MOD). Among the PVD techniques, the most commonly used methods for deposition of ferroelectric thin films are electron-beam evaporation, rf diode sputtering, rf magnetron sputtering, dc magnetron sputtering, ion beam sputtering, molecular beam epitaxy and laser ablation. PVD techniques require a high vacuum, usually better than 10.sup.-5 Torr. The advantages of PVD techniques are (1) dry processing, (2) high purity and cleanliness, and (3) compatibility with semiconductor integrated circuit processing. However, these are offset by disadvantages such as (1) low throughput, (2) low deposition rate, (3) difficult stoichiometric control, (4) high temperature post deposition annealing, and (5) high equipment cost.
The sol-gel and MOD processes for deposition of thin films are popular because of their simplicity. Additionally, they provide molecular homogeneity, high throughput, excellent compositional control and low capital cost since no vacuum is required. However, for ferroelectric thin films they are limited by film integrity problems during post deposition annealing and possible contamination problems that make them incompatible with semiconductor processing.
Of all the above mentioned techniques, metalorganic chemical vapor deposition technique (MOCVD) appears to be the most promising because it offers the advantages of simplified apparatus, excellent film uniformity, compositional control, high film densities, high deposition rates, excellent step coverage and amenability to large scale processing. The excellent film step coverage offered by MOCVD cannot be equaled by any other technique. Purity, controllability, and precision that have been demonstrated by MOCVD are competitive with molecular beam epitaxy (MBE). More importantly, novel structures can be grown easily and precisely. MOCVD is capable of producing an entire class of devices which utilize either ultra-thin layers or atomically sharp interfaces. In addition, different compositions can be fabricated using the same source.
Although MOCVD techniques are now being used to fabricate several demonstrative ferroelectric devices such as pyroelectric detectors, ultrasonic sensors, surface acoustic wave devices and several electro-optic devices, the primary impetus of recent activity in ferroelectric thin films is the large demand for commercial nonvolatile memories. As mentioned earlier, ferroelectric materials are characterized by a spontaneous polarization that can be reversed by reversal of the applied field. The polarization in the material shows hysteresis with the applied electric field; at zero field, there are two equally stable states of polarization, +P.sub.r or -P.sub.r, as shown in FIG. 1. This type of behavior enables a binary state device in the form of a ferroelectric capacitor (metal-ferroelectric-metal) that can be reversed electrically. Either of these two states could be encoded as `1` or `0` in a computer memory and since no external field (power) is required to maintain the state of the device, it can be considered a nonvolatile memory device. To switch the state of the device, a threshold field (coercive field) greater than +E.sub.c or -E.sub.c is required. In order to reduce the required applied voltage, the ferroelectric materials need to be processed in the form of thin films. Integration of ferroelectric thin film capacitors into the existing VLSI results in a true nonvolatile random access memory device (see J. F. Scott and C. A. Paz de Araujo, Science, 246, (1989), 1400-1405 ). In addition to the nonvolatility, ferroelectric random access memories (FRAMs) also offer high switching speeds, low operating voltage (&lt;5 V), wide operating temperature range and high radiation hardness. Furthermore, the ferroelectric thin films, electrodes and passivation layers can be deposited in separate small facilities thereby obviating the need for any changes in the existing on-line Si or GaAs VLSI production. In principle, FRAMs could eventually replace static RAMs (SRAMs) in the cache memory, dynamic RAMs (DRAMs) in the main system memory and electrical erasable programmable read only memories (EEPROMs) in look up tables.
Although ferroelectric thin films offer great potential for nonvolatile RAMs, commercial usage has been hindered largely by serious degradation problems such as fatigue, leakage current and aging that affect the lifetime of ferroelectric devices. A common source for these degradation properties in oxide ferroelectrics is the presence of defects such as oxygen vacancies in the materials. Considering the problem of fatigue, ferroelectrics are noted to lose some of their polarizability as the polarization state of ferroelectrics is repeatedly reversed. Fatigue (see I. K. Yoo and S. B. Desu, Mat. Sci. and Eng., B13, (1992), 319; I. K. Yoo and S. B. Desu, Phys. Stat. Sol., a133, (1992), 565; I. K. Yoo and S. B. Desu, J. Int. Mat. Sys., 4, (1993), 490; S. B. Desu and I. K. Yoo, J. Electrochem. Soc., 140, (1993), L133) occurs because of both the relative movement of oxygen vacancies and their entrapment at the electrode/ferroelectric interface (and/or at the grain boundaries and domain boundaries). These defects are created during the processing of ferroelectric films (with the desired ferroelectric phase) and can be classified into intrinsic and extrinsic defects. Extrinsic defects are the impurities that are incorporated in the films during processing and can be controlled by controlling the processing environment. Intrinsic defects can be divided into two types: (a) defects such as Schottky defects that maintain stoichiometry and (b) defects that alter stoichiometry in the materials. Examples of the formation of these defects can be illustrated by considering the case of PbZr.sub.x Ti.sub.1-x O.sub.3 (PZT) which is the most widely investigated ferroelectric thin film material for nonvolatile memory applications. Schottky defects in perovskite (ABO.sub.3) ferroelectrics such as PZT may be represented by a quasi-chemical reaction (in Kroger-Vink notation) as: EQU A.sub.A +B.sub.B +3O.sub.o .fwdarw.V.sub.A "+V.sub.B ""+3V.sub.o.sup.oo +A.sub.s +B.sub.s +3O.sub.s ( 1)
where A.sub.A, B.sub.B, and O.sub.o represent respectively occupied A, B and O sites; V.sub.A ", V.sub.B "", and V.sub.o.sup.oo represent vacancies of A, B and O atoms; and A.sub.s, B.sub.s, and O.sub.s are the respective Schottky defects. A typical example of defects that alter the stoichiometry are vacancies that are formed due to the vaporization of one or more volatile components in multicomponent oxide materials. In the case of PZT, for example, a processing temperature of at least 600.degree. C. is required to form the ferroelectric perovskite phase. However, the PbO component begins evaporating at temperatures as low as 550.degree. C., resulting in the formation of oxygen and lead vacancies as shown below: EQU Pb.sub.Pb +Ti.sub.Ti +Zr.sub.Zr +3O.sub.o .fwdarw.xPbO+xV.sub.Pb "+xV.sub.o.sup.oo +(1-x)Pb.sub.Pb +Ti.sub.Ti +Zr.sub.Zr +(1-x) O.sub.o( 2)
Intrinsic defects may also be created by stresses developed in the films during ferroelectric domain switching. It has been shown quantitatively [see S. B. Desu and I. K. Yoo, J. Electrochem. Soc., 140, (1993), L133] that relative migration of these oxygen vacancies and their entrapment at the electrode/ferroelectric interface (and/or at grain boundaries and domain boundaries) are important factors contributing to degradation in oxide ferroelectrics. The case of fatigue can be used to illustrate this point. As mentioned previously, fatigue in ferroelectric thin films is the loss of switchable polarization with increasing number of polarization reversals. Under an externally applied a.c. field (required to cause polarization reversal), the oxygen vacancies have a tendency to move towards the electrode/ferroelectric interface as a result of the instability of the interface. Eventually, these defects are entrapped at the interface and cause structural damage. This results in a loss of polarization in the material.
There are two possible solutions to overcome fatigue and other degradation problems. The first is to reduce the tendency for entrapment by changing the nature of the electrode/ferroelectric interface. Multilayer electrode structures using ceramic electrodes such as RuO.sub.2 which minimize oxygen vacancy entrapment have been used to minimize fatigue problems in oxide ferroelectrics (see U.S. patent application Ser. No. 08/104,861 for Multilayer Electrodes for Ferroelectric Devices, filed Aug. 10, 1993, the contents of which are hereby incorporated by reference). The second solution involves the control of defect density. The extrinsic point defect concentration may be minimized by reduction of impurity concentration or through compensation of impurities. La and Nb doping are known to reduce the fatigue rate of PZT thin films on Pt electrodes by compensating for the vacancies [see S. B. Desu, D. P. Vijay and I. K. Yoo, Mat. Res. Soc. Symp., 335, (1994), 53.]. The strategies for minimizing the intrinsic defect concentration may include choosing compounds with inherently high defect formation energies or choosing compounds that have no volatile components in their sublattice exhibiting ferroelectric properties. Thus, another alternative to overcome fatigue and other degradation problems is to use a ferroelectric compound that does not contain any volatile components in its sublattice that exhibits ferroelectric properties. This criterion is satisfied by many of the known layered structure ferroelectric oxides.
In the past, layered structure oxides have not been seriously considered as candidates for ferroelectric device applications. Attempts were made to use Bi.sub.4 Ti.sub.3 O.sub.12, which is a layered structure material, as a gate material on a transistor in a switching memory application (see S. Y. Wu, IEEE Transactions on Electron Devices, August 1974, pp. 499-504). However, the device showed early degradation and was therefore unsuitable for memory applications (S. Y. Wu, Ferroelectrics, 1976, Vol. 11, pp. 379-383). It is believed that development of any successful practical devices using layered structure oxides has been hindered by the inability to deposit high quality thin films of these materials.